Multiple radio frequency (RF) transmit coils are used for diverse magnetic resonance applications such as imaging or spectroscopy. For example, image quality can be improved by using multiple RF transmit coils in conjunction with shimming techniques by which the relative output powers of the various transmit coils are varied in a predetermined manner in order to enhance B1 field uniformity. In this way, multiple transmit coils can provide a relatively larger or more precisely shaped excitation volume with requisite B1 field uniformity as compared with what can be achieved using a single surface transmit coil or a whole body transmit coil.
These applications are predicated upon accurate knowledge of the B1 fields generated by the various transmit coils. It is known that the total B1 field generated by a combination of RF transmit coils is a linear combination or superposition (including both magnitude and phase) of the B1 fields generated by the individual RF transmit coils. The B1 field generated by each individual coil can be empirically quantified by acquiring a B1 field or flip angle map of the generated field in a suitable subject such as a phantom or the subject to be imaged. Due to the superposition principle, these individual coil B1 maps can be employed to determine an optimal combination of RF transmit coils for use in a given application.
A fast B1 mapping technique sometimes referred to as “Actual Flip Angle Mapping” or AFI is known. See Yarnykh et al., “Actual flip angle imaging in the pulsed steady state”, Proc. of the 12th Annual Meeting of ISMRM (Kyoto, Japan, 2004)(Abstract 194); Yarnykh, “Actual flip-angle imaging in the pulsed steady state: a method for rapid three-dimensional mapping of the transmitted radiofrequency field”, Magn. Reson. Med. vol. 57, pp. 192-200 (January 2007). AFI employs a dual repeat time (TR) steady state gradient echo sequence. The flip angle maps are derived from the images by a simple and robust approximation, which facilitates automatic evaluation on the scanner and in-vivo acquisition of 3D flip angle maps.
However, the performance of existing B1 field or flip angle mapping techniques is less than ideal. Error propagation and other difficulties in the case of small flip angles can be problematic, leading to noise in the B1 field map and potentially unreliable mapping of flip angles below about 15-20°. The AFI mapping technique employs large transverse magnetization spoiling gradients that increase the acquisition times of the AFI sequences, and sensitivity of the AFI mapping technique to gradient imperfections such as those generated by main magnetic field eddy currents can be problematic.
The following provides new and improved apparatuses and methods which overcome the above-referenced problems and others.